1. Field of the Invention
The present invention relates to a light emitting element device including a light emitting thyristor, and an optical print head including light emitting element devices.
2. Description of the Related Art
Optical print heads including a light emitting thyristor array (or light emitting element array) including multiple light emitting thyristors (or light emitting elements) are proposed as optical print heads (or exposure devices) of electrophotographic image forming apparatuses (see, for example, Japanese Patent Application Publication No. 2015-109417, in particular, FIG. 4 and paragraphs 0036 to 0060). Japanese Patent Application Publication No. 2015-109417 discloses a light emitting thyristor having an npnp layered structure in which a p-type anode layer (or first semiconductor layer), an n-type gate layer (or second semiconductor layer), a p-type gate layer (or third semiconductor layer), and an n-type cathode layer (or fourth semiconductor layer) are layered, a gate electrode being formed on the p-type gate layer (or third semiconductor layer). The n-type gate layer (or second semiconductor layer) has a band gap smaller than that of each of the p-type anode layer (or first semiconductor layer) and n-type cathode layer (or fourth semiconductor layer), and the p-type gate layer (or third semiconductor layer) has a band gap smaller than that of the n-type gate layer (or second semiconductor layer), so that the p-type gate layer (or third semiconductor layer) is a light emitting layer.
In the above layered structure, the surface on which the gate electrode is disposed is formed by wet etching the p-type gate layer. Thus, the p-type gate layer needs to have a sufficiently large thickness (or a thickness including an etching margin) in consideration of variation in etching rate of the wet etching. However, increase in thickness of the p-type gate layer, which is a light emitting layer, increases light absorption by the p-type gate layer itself, thus reducing light extraction efficiency. Further, increase in thickness of the p-type gate layer, which is a base layer of the upper npn structure (or npn transistor) of the light emitting thyristor, reduces the current gain of the npn transistor, thus deteriorating the switching characteristics of the light emitting thyristor.